1. Field of the Invention
The present invention relates to amplification circuit for electric charge type sensor used for amplifying and outputting the outputs of the electric charge type sensors such as acceleration sensors.
2. Description of the Related Art
There are conventionally known sensor apparatuses incorporating sensors in which detection signals are outputted in the form of electric charge, such as a piezoelectric type acceleration sensor apparatus and a pyroelectric type infrared-ray sensor apparatus. In such a sensor apparatus, a very small amount of electric charge is generated when the sensor detects an acceleration, an infrared ray, etc. In this case, for example, the amount of generated electric change is from 0.01 pC to a few thousands pC. Accordingly, an amplification circuit is used for amplifying the output of the sensor to extract as a voltage signal. In addition, the piezoelectric type acceleration sensor apparatus is utilized to detect vehicles"" air-bag operation (collision detection), angular acceleration generated when a vehicle spins, an impact on a hard disk drive, and the like.
A conventional amplification circuit which amplifies the output of a sensor with a charge amplifier is disclosed in the Japanese Unexamined Patent Application Publication No. 8-338781. FIG. 5 shows the structure of the conventional amplification circuit. The amplification circuit includes a resistor (a feedback resistor) R11 connected between the inversion input terminal of an operational amplifier Amp and the output terminal thereof. The resistor R11 is connected in parallel to the capacitor (a feedback capacitor) C11. One end of an acceleration sensor (G sensor) is connected to the inversion input terminal of the operational amplifier Amp, and the other end of the sensor is connected to a reference voltage Vref. The non-inversion input terminal of the operational amplifier is connected to the reference voltage Vref.
In the above amplification circuit, when vibrations are applied to the acceleration sensor, the acceleration sensor amplifies and outputs an electric charge Q corresponding to the magnitudes of generated acceleration and the vibrations. When the charge sensitivity of the acceleration sensor is expressed by d[pC/G], an output voltage Vout(G) with respect to an acceleration G will be obtained by the following equation. The symbol s is Laplace operator.                                                                         Vout                ⁢                                  (                  G                  )                                            =                              xe2x80x83                            ⁢                                                -                  d                                xc3x97                G                xc3x97                                                                                                        xe2x80x83                            ⁢                              (                                  1                                      C11                    xc3x97                                          (                                              1                        +                                                  1                                                      s                            xc3x97                            C11                            xc3x97                            R11                                                                                              )                                                                      )                                                                        [                  EQUATION          ⁢                      xe2x80x83                    ⁢          7                ]            
In the conventional amplification circuit, however, as will be described below, it is impossible to obtain an output Vout (the output of the operational amplifier Amp) in which a common mode noise overlapping with the output of the acceleration sensor is adequately cancelled. FIG. 6 shows a common mode noise mixture model in the conventional amplification circuit. In FIG. 6, the reference numerals Vn11, Cn11, Vn12, and Cn12 denote noise sources. Since the noise voltage Vn12 is connected to the reference voltage Vref via the capacitor Cn12, the noise voltage Vn12 has no influence on the output Vout of the operational amplifier Amp. In contrast, since the noise voltage Vn11 is connected to the inversion input terminal of the operational amplifier Amp via the capacitor Cn11, the noise voltage Vn11 has an influence on the output Vout of the operational amplifier Amp. As a result, the output Vout(N) of the operational amplifier Amp influenced by the common mode noise will be obtained by the following equation.                                                                         Vout                ⁢                                  (                  N                  )                                            =                              xe2x80x83                            ⁢                                                -                                                            s                      xc3x97                      Cn11                                        C11                                                  xc3x97                                                                                                        xe2x80x83                            ⁢                                                1                                      (                                          1                      +                                              1                                                  s                          xc3x97                          C11                          xc3x97                          R11                                                                                      )                                                  xc3x97                Vn11                                                                        [                  EQUATION          ⁢                      xe2x80x83                    ⁢          8                ]            
Based on Equations 7 and 8, there will be obtained a signal to noise ratio (S/N ratio) in the output Vout of the operational amplifier Amp as follows:                                                                         S                /                N                            =                              xe2x80x83                            ⁢                                                Vout                  ⁡                                      (                    G                    )                                                  /                                  Vout                  ⁡                                      (                    N                    )                                                                                                                          =                              xe2x80x83                            ⁢                                                d                                      s                    xc3x97                    Cn11                                                  xc3x97                                  G                  Vn11                                                                                        [                  EQUATION          ⁢                      xe2x80x83                    ⁢          9                ]            
On the other hand, as a circuit configuration capable of obtaining an output in which a common mode noise overlapping with the output of an acceleration sensor is properly cancelled, a differential amplification circuit is shown in FIG. 7. However, the circuit configuration is complicated, thereby increasing the production cost.
As one of techniques for obtaining outputs in which common mode noises overlapping with the outputs of acceleration sensors are adequately cancelled, Japanese Unexamined Patent Application Publication No. 5-188081 discloses a circuit configuration. A plurality of strip electrodes are disposed on a surface of a piezoelectric element (an acceleration sensor). The directions of polarization between the adjacent slip electrodes are opposite to each other. A ground electrode is disposed on the other surface of the piezoelectric element. In this case, whereas an amplification circuit amplifying a detection output of the acceleration sensor can have a simple circuit configuration, it is inevitable to use the piezoelectric element having a complicated element structure. As a result, there is a problem in that the production cost increases.
Accordingly, it is an object of the present invention to provide an amplification circuit for electric charge type sensor having a simple circuit configuration in which a common mode noise can be adequately cancelled.
According to a first aspect of the present invention, there is provided an amplification circuit for electric charge type sensor including an operational amplifier having an inversion input terminal, a non-inversion terminal, and an output terminal, the inversion input terminal being connected to one end of an electric charge type sensor and the non-inversion input terminal being connected to the other end of the sensor, a negative feedback circuit including a feedback resistor connected between the output terminal and the inversion input terminal and a feedback capacitor connected in parallel to the feedback resistor, and a cancellation circuit including a resistor connected between the non-inversion input terminal and a reference voltage and a capacitor connected in parallel to the resistor.
In the above circuit configuration, by using the cancellation circuit connected between the non-inversion input terminal of the operational amplifier and the reference voltage, an output of the operational amplifier is obtained, in which a common mode noise overlapping with the output of the electric charge type sensor is adequately cancelled.
According to a second aspect of the present invention, there is provided an amplification circuit for electric charge type sensor including an operational amplifier having an inversion input terminal and a non-inversion input terminal, the inversion input terminal being connected to one end of an electric charge type sensor and the non-inversion input terminal being connected to the other end of the sensor, a voltage divider having a voltage dividing point for dividing a voltage output from the operational amplifier, a negative feedback circuit including a feedback resistor connected between the inversion input terminal and the voltage dividing point and a feedback capacitor connected in parallel to the feedback resistor, and a cancellation circuit including a resistor connected between the non-inversion input terminal and a reference voltage and a capacitor connected in parallel to the resistor.
In this circuit configuration, there can be obtained the same advantages as those obtained in the structure of the first aspect of the invention. In addition, the voltage divider for dividing a voltage output from the operational amplifier permits an amplification rate as the sensitivity of the operational amplifier to be easily adjusted.
Furthermore, the negative feedback circuit may have the same circuit constant as the cancellation circuit has.
In addition, a line connecting the one end of the electric charge type sensor to the inversion input terminal may be arranged in the vicinity of a line connecting the other end of the sensor to the non-inversion input terminal.
In these circuit configurations, a common mode noise overlapping with an output of the piezoelectric sensor can be effectively cancelled .